It is known that, in image pickup elements, such as CCD sensors and CMOS sensors, a localized sensitivity failure of a semiconductor may occur during a manufacturing process or after the manufacturing process. When such a sensitivity failure occurs, an electric charge output in accordance with an incident light quantity cannot be obtained from a pixel, as a result of which a white spot or a black spot that is unrelated to an object appears on an image pickup screen. Such a pixel that causes a white spot or a black spot unrelated to an object to be output is called a defective pixel. In order to correct image quality degradation caused by such a defective pixel by signal processing, the defective pixel is detected beforehand. First, when manufacturing an image pickup element at a semiconductor factory, any defective pixel in the manufactured image pickup element is detected, and position data of the detected defective pixel is stored in a nonvolatile memory.
Even after installing the image pickup element in an image pickup apparatus, any defective pixel in the image pickup element can be detected. For example, when a mechanical shutter of the image pickup apparatus is in a light-shielding state, a pixel (white-spot defective pixel) whose output level from the image pickup element exceeds a predetermined level is detected. Alternatively, when the shutter is opened, and the incident light quantity is set to a predetermined quantity, any pixel (black-spot defective pixel) at which an output level does not reach the predetermined level is detected. Position data of the detected white-spot defective pixel or the detected black-spot defective pixel is stored in the nonvolatile memory. Because position data of the above-described defective pixels can be stored prior to normal image pickup operation (prior to the imaging apparatus being used for actual imaging of an object), these defective pixels can be referred to as “steady defective pixels”. During normal image pickup operation of the image pickup apparatus, an image signal obtained by imaging an object is corrected with signal processing by taking into account the defective pixels on the basis of the pre-stored position data.
In recent years, the probability with which defective pixels occurs tends to increase due to an increase in the number of pixels of image pickup elements. Further, formation of finer pixels resulting from the increase in the number of pixels of the image pickup elements has caused the recognition of new phenomena that have been hitherto overlooked. For instance, the existence of pixels whose signal levels are read increase or decrease considerably when pixel signals from the image pickup elements are repeatedly read out. Pixels that cause such a phenomena to occur are called blinking defective pixels. There are blinking defective pixels that depend upon temperature and storage time, and blinking defective pixels that do not depend upon temperature and storage time. Blinking defective pixels are variously mechanically generated.
Blinking defective pixels are normal pixels at certain times, and are white-spot defective pixels at other times, so that they act as though they are blinking white-spot defective pixels. Therefore, when a manufacturing process of an image pickup element is performed or when an image pickup apparatus performs a self-measurement operation, all of the blinking defective pixels cannot be detected by detecting each defective pixel once. In addition, the defective pixels are turned on during an actual image taking operation in which an image of a taken object is recorded, as a result of which the blinking defective pixels stand out, thereby degrading the taken image.
In view of such a situation, Japanese Patent Laid-Open No. 2003-37781 discloses a technology in which, on the basis of a plurality of image signals obtained under the same condition, defective pixel addresses of an image pickup element are detected, and the pixel addresses where the number of times by which defects are determined is greater than a predetermined number of times are detected as final defective pixel addresses.